1. Field of the Invention
The present invention relates to an abnormality determination apparatus and an abnormality determination method for a valve, and particularly to a technique of determining abnormality of a valve provided to an intake passage of an internal combustion engine.
2. Description of the Background Art
A tumble control valve (also referred to as TCV hereinlater) that strengthens a tumble flow in a cylinder has conventionally been known. The tumble flow strengthened by the TCV can gather fuel around a spark plug for example. Therefore, by operating an internal combustion engine under a considerably lean air-fuel ratio condition, the emission property can be improved. Since the TCV has a great influence on the emission property, it is necessary to determine, using an OBD (On Board Diagnostic system), whether or not the TCV is abnormal.
Japanese Patent Laying-Open No. 10-121991 discloses a fault diagnostic apparatus for an engine intake control system that controls the intake airflow by opening/closing an intake control valve provided to an engine intake system based on the engine operating state and that corrects the quantity of injected fuel by increasing or decreasing the fuel quantity for acceleration or deceleration. The fault diagnostic apparatus for the engine intake control system disclosed in the above-referenced patent document includes diagnostic means for determining that the intake control system fails when the air-fuel ratio has, in the period during which the fuel quantity is corrected by increasing it or the period during which the fuel quantity is corrected by decreasing it, any value that it never has during normal operation of the intake control valve.
The fault diagnostic apparatus disclosed in the patent document determines that the intake control system fails when the air-fuel ratio has, in the period during which the fuel quantity is corrected by increasing it for acceleration or the period during which the quantity is corrected by decreasing it for deceleration, a value that it never has during normal operation of the intake control valve. Under the conditions that the intake control system fails and the intake control valve is closed at the time of acceleration at which the operation changes from low-load operation to high-load operation, the charging efficiency is decreased due to the intake air resistance resultant from the fact that the intake control valve is closed so as to cause the quantity of fresh charge taken into the combustion chamber to decrease as compared with normal operation. At this time, the air-fuel ratio is rich and has any value that never occurs while the intake control valve normally operates. In contrast, under the conditions that the intake control system fails and the intake control valve is opened at the time of deceleration at which the operation changes from high-load operation to low-load operation, the charging efficiency is increased due to the fact that the intake control valve is opened to cause the quantity of fresh charge taken into the combustion chamber to increase. At this time, the air-fuel ratio is lean and has any value that never occurs while the intake control valve normally operates. Therefore, by determining the air-fuel ratio in the period during which the fuel quantity is increased for acceleration or the period during which the fuel quantity is decreased for deceleration, failure of the intake control system can be diagnosed.
The internal combustion engine of the common automobile has a plurality of cylinders. In such an internal combustion engine, each cylinder is provided with at least one TCV. The air-fuel ratio sensor detecting the air-fuel ratio does not detect the air-fuel ratio of each cylinder but detects the air-fuel ratio of the internal combustion engine as a whole. Therefore, if only one of a plurality of TCV is abnormal, there is a small influence on the air-fuel ratio detected by the air-fuel ratio sensor. Therefore, in the case where the failure of the TCV is determined from the air-fuel ratio as done by the failure diagnostic apparatus disclosed in Japanese Patent Laying-Open No. 10-121991, abnormality of any one of a plurality of TCV may not be detected in some cases.